This application is based on applications Nos. H09-305298 and H09-307301 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image pickup apparatus that employs as an image pickup device an image sensor of the type in which each pixel (picture element) generates a signal that corresponds to one of three primary colors.
2. Description of the Prior Art
In an image pickup apparatus, it is inevitable that the obtained image signals contain noise for various reasons, and therefore, to obtain cleanest images possible, it is essential to remove such noise. In an image pickup apparatus that employs as an image pickup device an image sensor of the type in which adjacent pixels generate signals corresponding to different colors, the most significant cause of noise is crosstalk between adjacent pixels. The principle of how noise is caused by crosstalk is as follows.
When, in an analog signal processing circuit, there is a difference between the levels of the image signals output from adjacent pixels that are read out consecutively, the image signals affect each other, causing their levels to deviate from their original levels. This phenomenon is referred to as xe2x80x9ccrosstalkxe2x80x9d in the present specification. Crosstalk increases as the signal level difference between adjacent pixels is greater.
Thus, the effect of crosstalk varies as the difference between the levels of the image signals output from adjacent pixels varies, and this is the cause of what is referred to as xe2x80x9cnoise due to crosstalkxe2x80x9d.
Now, with reference to FIGS. 1, 2A, and 2B, how noise due to crosstalk arises will be described specifically. FIG. 1 illustrates a mosaic filter as is typically bonded over the image-sensing surface of an image sensor. In this mosaic filter, filter elements for each of three primary colors, i.e. green (G), blue (B), and red (R), are arranged alternately in a two-dimensional array. When regarded as being composed of horizontal lines, the array is composed of RG lines (each composed of R and G pixels arranged alternately) and GB lines (each composed of G and B pixels arranged alternately) arranged alternately. The G pixels are arranged in a checkered pattern. In this image sensor, each pixel generates a signal corresponding to the color of the filter that is placed over that pixel. The electric charges occurring in the individual pixels of the image sensor are read out successively along one horizontal line after another, i.e. along the RG and GB lines alternately. This type of primary color filter tends to cause a large difference in the level of the image signal in particular between adjacent pixels, and thus tends to cause crosstalk.
When the image signals obtained along two consecutive RG and GB lines have waveforms as shown at (a) in FIG. 2A, the signal level difference between adjacent pixels is almost equal on the RG line and on the GB line, and therefore the G pixels are affected by crosstalk to an almost equal degree on these two lines. Thus, the deviation W of the levels of the G-pixel signals from the black level 80 is almost equal on these two lines. Interpolating the G pixels of each of these two lines by using the average of adjacent pixels yields a waveform as shown at (b) of FIG. 2A.
On the other hand, when the signal level difference between adjacent pixels on the RG line differs greatly from that on the GB line, and the image signals obtained along those two lines have waveforms as shown at (a) in FIG. 2B, the G pixels are affected by crosstalk to different degrees on those two lines. That is, on the GB line, where there is a large signal level difference between adjacent pixels, the G pixels are affected greatly by crosstalk from the B, pixels, and thus the level of the G-pixel signals is shifted upward, with its deviation W from the black level 80 increased. By contrast, on the RG line, the deviation W of the signal level of the G pixels is smaller than on the GB line.
In this case, interpolating the G pixels of each of the two lines by using the average of adjacent pixels, i.e. interpolating the portions occupied by the signals of the R (in the case of the RG line) or B (in the case of the GB line) pixels existing between the G pixels with the signals of appropriate G pixels (for example, with the average of the G pixels that are vertically adjacent to those R or B pixels and thus belong to the upper and lower adjacent lines), yields a waveform as shown at (b) in FIG. 2B, which indicates that the G-pixel signals as a whole suffer from dot-shaped noise. This noise is one type of noise that results from crosstalk.
Even in cases where the G pixels are not interpolated, there exists a difference in the signal level of the G pixels between the RG and GB lines (although no such difference should exist except at a border between different colors). Accordingly, when, for example, the image signals obtained from the signals shown in FIG. 2B are fed to a display, the image displayed thereon suffers from noise that appears as a pattern of alternating deeply and lightly colored lines. This noise is another type of noise that results from crosstalk.
One conventional way to avoid such noise is to reduce the color modulation factor of the entire image by filtering. This helps reduce the difference between the signal levels of adjacent pixels and thereby reduce the effect of crosstalk. This in turn leads to reduction of the noise that results from, variation in the effect of crosstalk and that is included in the image signals obtained after interpolation.
However, this method, although effective in preventing noise, cannot be adopted without degrading the definition of the entire image. Reducing the resolution of the image is considered to be another way to reduce the effect of crosstalk and thereby reduce noise.
As described above, it has conventionally been impossible to eliminate noise due to crosstalk without sacrificing the resolution and definition of the obtained image. Oil the other hand, using high-performance circuits that offer satisfactory high-frequency response helps reduce the effect of crosstalk, but this requires unduly high extra cost.
An object of the present invention is to provide an image pickup apparatus that can eliminate noise due to crosstalk without unduly degrading color rendering, without sacrificing resolution, and without requiring unduly high cost.
To achieve the above object, according to one aspect of the present invention, an image pickup apparatus is provided with: an image sensor having first-type, second-type, and third-type pixels each producing a signal corresponding to a first, a second, and a third color respectively, the pixels being arranged in a two-dimensional array consisting of first-type lines and second-type lines arranged alternately, the first-type lines each consisting of first-type and second-type pixels arranged alternately, the second-type lines each consisting of first-type and third-type pixels arranged alternately; a first subtracter for calculating a difference between the outputs of the first-type pixels and the outputs of the second-type pixels on the first-type lines; a second subtracter for calculating a difference between the outputs of the first-type pixels and the outputs of the third-type pixels on the second-type lines; and a control circuit for driving the image sensor in accordance with the outputs of the first and second subtracters.
According to another aspect of the present invention, an image pickup apparatus is provided with: an image sensor having first-type, second-type, and third-type pixels each producing a signal corresponding to a first, a second, and a third color respectively, the pixels being arranged in a two-dimensional array consisting of first-type lines and second-type lines arranged alternately, the first-type lines each consisting of first-type and second-type pixels arranged alternately, the second-type lines each consisting of first-type and third-type pixels arranged alternately; a color temperature detecting circuit for detecting the color temperature of the light coming from an object to be shot; and a control circuit for driving the image sensor in accordance with the output of the color temperature detecting circuit.
According to still another aspect of the present invention, in a method of reproducing an image, an image sensor is used that has first-type, second-type, and third-type pixels each producing a signal corresponding to a first, a second, and a third color respectively. In this image sensor, the pixels are arranged in a two-dimensional array consisting of first-type lines and second-type lines arranged alternately, with the first-type lines each composed of first-type and second-type pixels arranged alternately and the second-type lines each composed of first-type and third-type pixels arranged alternately. Moreover, this method includes the steps of: calculating the difference between the outputs of the first-type pixels and the outputs of the second-type pixels on the first-type lines; calculating the difference between the outputs of the first-type pixels and the outputs of the third-type pixels on the second-type lines; and driving the image sensor in accordance with the calculated two differences.
According to a further aspect of the present invention, an image pickup apparatus is provided with: an image sensor having first-type, second-type, and third-type pixels each producing a signal corresponding to a first, a second, and a third color respectively, the pixels being arranged in a two-dimensional array consisting of first-type lines and second-type lines arranged alternately, the first-type lines each consisting of first-type and second-type pixels arranged alternately, the second-type lines each consisting of first-type and third-type pixels arranged alternately; a first subtracter for calculating a difference between the outputs of the first-type pixels and the outputs of the second-type pixels on the first-type lines; a second subtracter for calculating a difference between the outputs of the first-type pixels and the outputs of the third-type pixels on the second-type lines; and a correction circuit for correcting the outputs of the first-type pixels in accordance with the outputs of the first and second subtracters.